Method for electrostatic spraying of non-conductors



United States Patent Grant E. Arhart, Greensboro, N.C.,

a corporation Chemical Coatings, Inc., Chicago, 11].,

of Delaware N Drawing. Filed May 2, 1958, Ser. No. 732,429

7 Claims. (Cl. 117-215) My invention relates to methods of coatingarticles formed of non-conducting materials, such as wood, fibrousmaterial, etc., by electrolytic spray means, whereby certain new andimproved results are obtained.

I realize that, broadly considered, electrolytic spray coating has beenpracticed for some time and apparatus adaptable for this purpose hasbeen and is being marketed by Ransburg Electro-Coating Corporation ofIndianapolis, Indiana, certain of which equipment may be adapted for thepractice of my invention. For example, Starkey United States Patent No.2,723,921, dated November 15, 1955, describes one of such previouslyknown methods of electrolytic or electrostatic coating, of which myinvention may be considered an improvement.

I am also familiar with Cross Patent 2,281,558, dated May 5, 1942, whichhas to do with the manufacture of sandpaper by a method which utilizesdeposition of material on a web of paper in an electrostatic field.

A particular object of my invention is the application of a coating to abase of non-conducting material by an electrostatic method wherebyeither a clear or pigmented coating may be applied economically andefliciently, utiliZing a minimum of coating material. An importantadvantage of my invention is that it makes possible the coating of woodor other non-conductive material having an attractive grain, color ortexture with a clear coating whereby such features of the coated basematerial remain visible in the finished coated article. I believe thisto be novel in,my invention.

Another important advantage of my invention is that it makes possiblethe coating of non-conductive material with a single relatively thincoat.

The above mentioned objectives are not possible in any of the previouslyknown methods of this type with which I am familiar.

Broadly considered, in accordance with my invention, the non-conductivesurface is first coated with a mixture or solution of resin, with orwithout pigment, and an aqueous solution of electrolyte; after drying,the article is then electrostatically coated with either a clear orpigmented organic coating.

More specifically, my invention is practiced by first applying to thenon-conductive surface, as by dipping, brushing, spraying or equivalentmethod, a mixture or solution of resinous material in a suitable solventor solvents with an aqueous solution of phosphoric acid, which has beenfound vastly superior to any other electrolyte which I have tried. Afterair drying, the article is then passed through a suitable Ransburgelectrostatic sprayer, as for example a machine known as Ransburg No. 2,or other suitable equipment of this character, where it is spray coatedwith a desired organic coating, which coating is likewise air dried orforced dried. If desired, an additional coating may subsequently beapplied electrostatically as a top coat following a graining operation.

Example No. 1

Percent Polyvinyl butyral resin 7.0 Isopropyl alcohol 73.5 Xylol 15.5Phosphoric acid 2.0 Water 2.0

The xylol referred to in the formula is a commercial mixture of ortho,meta and para xylene and is employed for its solvent value to facilitatethe application of the mixture to the base surface. It may be replacedby other suitable solvents and, in fact, is not an indispensableconstituent.

The alcohol serves as a distributing agent for the resin and is of valuein the formula in rendering the water miscible with the otherconstituents. Other suitable solvents known to the chemist skilled inthis field may be substituted.

The resin solids included in the above formula serve as a sealer of theporous substrate and aid in holding or scaling in the electrolyticsolution until such time as the article is electrostatically sprayed.

Other resins may be substituted for the one referred to above, thesolvent balance being modified according to normal practice in this art,in order, to accommodate the difference in solubility characteristics ofthe various resins. As examples of other resins which may be employed Imight mention the vinyl acetates, the acrylic resins, the alkyd resins,the cellulose resins, the epoxy resins, the hard resins such as esterresins, hydrocarbons, maleics, phenolics, and such natural resins as mayhave properties suitable for the purposes here described. Nitrogenousresins, oleo-resinous varnishes and polyesters having the desiredproperties, as known to those skilled in the art, may also be employed.

The mixture obtained by the above formula is applied as by spraying, tothe surface of a non-conductive article such as wood, and after airdrying for about 15 minutes the article so coated is placed on aconveyor and passed before a Ransburg No. 2 station or the like. At thispoint there is applied a clear coat such as the following:

Percent Half-second butyrate 11 Synthetic hard resin 13 Plasticizer 3Thinners 73 Suitable examples of the constituents referred to above willbe obvious to those skilled in the art.

Another suitable clear coating may be represented by the followingformula:

Percent Synthetic alkyd resin solids 10 Hard resin solids 10 Chlorinatedrubber 10 Solvents and driers 69 Plasticizer 1 Example N0. 2

Percent Titanium dioxide 1.6 Aluminum silicate 4.1 Polyvinyl butyralresin 5.6 Isopropyl alcohol 80.0 Xylene 1.5 Water 4.3 Phosphoric acid2.9

The mixture obtained by the above formula is applied to thenon-conductive article as by a spray gun, allowed to dry in the air andthe article thus coated is passed before a Ransburg No. 2 station wherea pigmented organic coating is applied as represented by the followingformula:

Percent Color pigments 11 Extender pigments 9 Synthetic alkyd resinsolids 15 Synthetic chlorinated rubber 14 Thinner and driers 51 Anotherexample of a pigmented coating is:

Percent Titanium dioxide 30 Extender pigment 16 Synthetic alkyd resinsolids 9 Chlorinated rubber 7 Solvents and driers 38 Example N0. 3

Percent Polyvinyl butyral resin 7.2 Zinc chromate 6.9 Magnesium silicate1.0 Butyl alcohol 16.1 Ethyl alcohol 62.0 Phosphoric acid 3.6 Water 3.2

The mixture represented by the above formula is applied by spray gun orotherwise to the non-conductive article and allowed to dry in the air.The article is then hung on a conveyor line and passed before a RansburgNo. 2 station where it is coated with a pigmented coating such asspecified above, or other suitable formula.

As stated above, the process constituting my invention has importantadvantages over previously known electrostatic coating methods. I findthat satisfactory coatings and finishes may be applied with a minimum ofmaterials, a second coat frequently being found unnecessary.Furthermore, clear coatings may be applied, permitting the color andgrain of the base material to show through.

I am aware that carbon black and other solids have been used asconductors in electrostatic coating methods previously known and suchmaterials have important disadvantages as compared with my invention.Such solids completely conceal the color, grain, etc., of the basematerial and furthermore require a heavier coating of the appliedcoating material to conceal the color of said conductive materials.Furthermore, I find that use of the electrolytic conductors according tomy invention act more efficiently in attracting the coating materialelectrostatically applied, and thus effect economies.

Various other modifications of my invention may suggest themselves tothose skilled in the art Without departing from the spirit of myinvention. Hence, I do not wish to be limited .to the specific examplesor illustrations described above, except to the extent indicated in theappended claims.

I claim:

1. A method of coating a normally non-conductive base article,comprising applying to the surface of said article a coating materialdistributed in a liquid medium containing in solution a relatively smallportion of phosphoric acid, drying, and thereafter spraying said coatedarticle in an electrostatic field with a second coating.

2. A process as in claim 1 wherein the second coat is sufliciently clearto permit substantial visibility therethrough.

3. A process as in claim 1 wherein both coats are sufi'iciently clear topermit visibility of the base material in the finished article.

4. A process as in claim 1 wherein the first coat comprises asubstantially clear-drying composition of a resin dispersed in a solventtherefor, the phosphoric acid is in aqueous dilution, and an alcohol ispresent.

5. A process as in claim 1 wherein the second coat is pigmented.

6. A process as in claim 1 wherein both of the coatings are pigmented.

7. A method of coating wood, to render the grain of the wood visible inthe finished article, comprising applying to the surface of the articlea coating composition comprising substantially clear-drying resindispersed in organic liquid solvent and distributing media, said coatingcomposition including a minor proportion of phosphoric acid and water,drying the article so coated, and thereafter spraying said coatedarticle in an electrostatic field with a second coating, the latterbeing likewise clear drying.

References Cited in the file of this patent UNITED STATES PATENTS1,881,445 Flanzer et a1. Oct. 11, 1932 2,583,000 Lylte Jan. 22, 19522,617,742 Olson Nov. 11, 1952 2,698,814 Ransburg Ian. 4, 1955 2,719,820Allen Oct. 4, 1955 2,723,921 Starkey Nov. 15, 1955 2,761,797 Young Sept.4, 1956 2,762,726 Saunders Sept. 11, 1956 2,772,190 Haayman et a1. Nov.27, 1956

1. A METHOD OF COATING A NORMALLY NON-CONDUCTIVE BASE ARTICLE,COMPRISING APPLYING TO THE SURFACE OF SAID ARTICLE A COATING MATERIALDISTRIBUTED IN A LIQUID MEDIUM CONTAINING IN SOLUTION A RELATIVELY SMALLPORTION OF PHOSPHORIC ACID, DRYING, AND THEREAFTER SPRAYING SAID COATEDARTICLE IN AN ELECTROSTATIC FIELD WITH A SECOND COATING.